A war in North Korea could break out 'any time between now and March 2018', a Chinese former senior military official has warned.

Lieutenant General Wang Hongguang told Chinese state media that Beijing should be mentally prepared for war on the Korean peninsula at any moment, and should start mobilising troops in Northeast China.

His warning came the day before China sent several warplanes on a long-range drill to the Sea of Japan, a move that prompted South Korea to scramble fighter jets.

Time to prepare: Lieutenant General Wang Hongguang, a former senior military official said that a war on the Korean peninsula is imminent

'The war on the Korean Peninsula might break out anytime between now and March next year,' General Hongguang told Chinese state-run English language newspaper Global Times.

'China should be psychologically prepared for a potential Korean war, and the Northeast China regions should be mobilized for that.'

'Such mobilization is not to launch a war, but for defensive purposes,' he added.

Monday's military drill saw the Chinese air force dispatched bombers, fighters and reconnaissance planes through the Tsushima Strait to the Sea of Japan to 'test its ocean combat ability.'

'This is a regular annual training arrangement of China's air force that accords with the relevant international laws and practices and it isn't aimed at any particular state, region and target,' air force spokesman Shen Jinke said in a statement.

Warning: Lieutenant General Hongguang said his advice is that Beijing should start mobilizing troops in Northeast China

This was the first time the Chinese air force has flown through the strait that lies between South Korea and Japan, Shen said.

Seoul's Joint Chiefs of Staff said five Chinese warplanes – two bombers, two fighter jets and one reconnaissance plane – entered South Korea's air defense identification zone off a southern South Korean island almost at the same time on Monday.

They then flew to the Japanese air defense zone above the waters between the Korean Peninsula and Japan, a JCS official said on condition of anonymity, citing department rules.

The official said the Chinese planes entered the South Korean air defense zone again before returning to China.

South Korea used a military hotline to warn China of its planes' entrance to the zone and China replied that it's part of routine training, the official said.